Probing higher resolution: an asymmetry in the Chandra PSF

Contents

Abstract

As part of efforts to push the spatial resolution of Chandra to
the sub-ACIS-pixel regime, we have identified a feature in the
Chandra/HRC point spread function within the central arcsecond
which may affect high fidelity deconvolutions. There is
evidence that the feature is also present in Chandra/ACIS
data. The problem does not affect images on scales larger than
one arcsecond.

What do we know?

There is an asymmetry in the HRC images at ~0.6-0.8" along
the negative HRC V direction, which is also the negative CHIPY
direction in HRC event lists. (Fig
1, Fig 3).

The feature's brightness appears to be ~5% (and perhaps somewhat
higher) of the total brightness.

The asymmetry (a hook-like structure) seems to be fixed in the
spacecraft coordinate and extends roughly between mirror
spherical coordinate azimuthal angle, PHI, of 260-310
deg. PHI is measured counterclockwise from the +DETX (Spacecraft
+Y) axis; this range corresponds to a position angle of 170-220
deg measured counterclockwise from the +DETY (Spacecraft +Z) axis.
(The difference
between Fig 1, Fig
2 and the Fig 3 is that
the first two are in the HRC instrumental U-V coordinates,
and Fig 3 is in spacecraft
coordinates.)

Residuals in 2D fits to ACIS data persistently show enhancements
relative to the PSF in the +Z direction that are consistent with
the PSF hook-like feature.

The asymmetry was discovered using on-axis AR Lac observations
(Fig 1, Fig
3), and was confirmed using HRC observations of
Capella (Fig 2) and other point
sources. Techniques used include HRMA PSF fitting and analysis
of residuals; deconvolution using simulated HRMA (ChaRT/SAOTrace
PSFS); PSF subtraction (e.g. Fig 1
and Fig 2, Fig
3)

A time series of AR Lac observations obtained almost yearly
since October 1999 shows that the asymmetry (with its present
characteristics) appeared sometime between October 1999 and
December 2000 (Fig 1) and is
present in the recent observations obtained in 2010 December. It
appears to be stable since December 2000.

What do we suspect?

Analysis of several ACIS images (SER, deconvolution,
subtraction) indicate that there is an asymmetry in a similar
direction and spatial scale as in the HRC images
(Fig 4). The
figure
of panda counts ratios from Kashyap shows the asymmetry
in the counts in the data along +Z compared to counts along
-Z.

The analysis of the ACIS data is more complex because of its
coarser pixel scale and thus coarser sampling of the
mirror+instrument PSF. If the asymmetry is indeed in the ACIS
data as well then it would suggest that the origin of this
asymmetry is in the HRMA mirrors. The CXC is continuing its
investigation.

Advice to users

The make_psf_asymmetry_region tool
creates a region file that identifies the area where the
asymmetry may be affecting the analysis (especially for a 2-D
analysis looking for faint jets, shells, or similar structures).
To obtain make_psf_asymmetry_region,
download the CIAO scripts package
(most CIAO installations will have this installed).

Note that although the feature is fixed in detector coordinates,
it is not possible to inspect it by binning on DETX, DETY since
its offset from the PSF core is completely swamped by the
dither. To look for evidence of the feature, make a sky
coordinate image using small pixels, e.g. with the binning

[sky=region(src.reg)][bin x=::0.2,y=::0.2]

The position angle (PA) of a feature on the sky is measured from N
through E in the standard astronomical convention; so a feature
towards SKY +Y has PA = 0, and towards SKY +X has PA = 270 deg.
The PA is related to the detector mirror spherical coordinate
PHI by

PA = PHI - ROLL - 90 deg

and so the PSF artifact, which is at 285 +/- 25 deg in PHI, is
in the sky direction

Users should treat with suspicion any extent in their images at
scales less than 1 arcsecond in a direction of 260-310 deg (in
spacecraft MSC coordinates) with a flux less than 10 percent of
the total brightness of the source. Two observations at
different spacecraft rolls will be advisable for searches for
shells or jets at scales of less than 1".

Simulations suggest that encircled energy analyses would show
detectable effects of this asymmetry for sources with several
hundred counts or more.

Figures

Deconvolution of observed source image with ray-trace image
for the AR Lac calibration observations. The vertical white
bar is 1 arcsec in length. By the 2002 observation a
hook-shaped extension to the -V direction has appeared.
This feature appears to be stable over the last several
years. The orientation is in HRC-I instrumental U-V coordinates.

Similar to Figure 1 but for Capella calibration observations
that translated the HRC-I behind the telescope
aim-point. The offset position is indicated above each
panel. The offsets are large enough that the dither samples
non-overlapping regions of the detector. The same hook-like
feature as seen in the AR Lac data is observed.
The orientation is in HRC-I instrumental U-V coordinates.

Left - AR Lac HRC-I observation (2009 Sep 24, ObsID 10578)
deconvolved with the simulated PSF showing several
sub-arcsecond features including a significant asymmetry, an
arc-like structure (6% from the total brightness), centered
at ~200deg at 0.8", measured counterclockwise from the +DETY
axis. The image was smoothed for visual
purpose, to show better the asymmetry. The image has been
rotated through the roll angle so that the axes are parallel
to DETX,DETY (ignoring the effects of dither).

Right - The PSF subtracted original AR Lac observation
showing similar structures as in the deconvolved image, and
especially the arc-like asymmetry at ~0.8". Additional
asymmetries are noted but at a scale less then 0.5" which
require further investigation. Similarly to the deconvolved
image , the subtracted image has been smoothed for visual
purposes.

Left - ACIS-I tau CMa observation deconvolved with the
simulated PSF smoothed with gauss (pixel size 0.1") showing
asymmetry in the same direction and spatial scale as in the
HRC-I observation to the right.